The present invention relates to computer systems. More specifically the present invention relates to a method and apparatus for matched length routing for connecting land pads of components placed back-to-back on a printed circuit board in a computer system.
Computer systems often include components that are mounted on double sided printed circuit boards. When such components have identical (non-mirrored) pin-outs and require their corresponding contacts to be connected, internal layers of the printed circuit boards are typically utilized to provide a routing path. When routing components with identical pin-outs back-to-back on double sided printed circuit board, the signal line routed from a land pad connected to a first contact of a first device on a primary side of the printed circuit board to a land pad connected to a corresponding first contact of a second device on the secondary side differs in length from the signal line routed from a land pad connected to an adjacent second contact on the first device to a corresponding land pad connected to an adjacent second contact of a second device. The difference can be calculated as being approximately two times the package land pitch. The package land pitch may be defined as the distance measured between contacts of a component.
The length of a signal line between the contacts of two devices may be measured by the stub lengths on each side of the printed circuit board and the pitch between the devices. The stub length may be defined as the distance between a land pad connected to a contact of the component and a xe2x80x9cTxe2x80x9d point of a via. The pitch may be defined as the distance of the trace connecting the vias connected to the stubs. The length of a signal line determines the electrical delay and capacitance on the signal line. Differences in electrical delay and capacitance between signal lines may adversely effect the performance of components operating at high speeds.
A printed circuit board is disclosed. The printed circuit board includes a first component mounted on a first side of the printed circuit board. A second component has an identical pin-out as the first component. The second component is mounted on a second side of the printed circuit board. A first signal line connects a first landpad coupled to a first contact on the first component with a second landpad coupled to a corresponding first contact on the second component. A second signal line connects a third landpad coupled to a second contact on the first component with a fourth land pad coupled to a corresponding second contact on the second component. The first signal line is equal in length to the second signal line.